The present invention relates to a linear digital filter which is nonrecursive and can thus be characterized by an impulse response of finite length, particularly for filtering signals in which identical values frequently appear in succession. Such signals are produced, for example, by sampling an analog signal having a step function shape.
One field of use for such a filter, for example, is the demodulation of frequency shift keyed signals (FSK modulation) with the aid of a zero crossing discriminator. For each zero crossing of the FSK signal, a rectangular pulse is generated which, in the simplest case, has a given length in time. Steeper static demodulator characteristics are obtained by the use of zero crossing discriminators of a higher order, in which the length of the pulse depends on the time spacing of the the zero passages, as disclosed by Dr. G. Ruopp in "Auswertung frequenzmodulierter Signale mit Nulldurchgangsdiskriminatoren" [Evaluation of Frequency Modulated Signals By Means Of Zero crossing Discriminators", in FREQUENZ [Frequency], Vol. 32, 1978, pages 227-232. The useful signals which can be fed directly to a threshold decider, is obtained by filtering the binary pulse train through a lowpass filter. Using continuous-time filter technology, such lowpass filters are realized in the form of passive LCR filters or active RC filters which have the significant drawback that they frequently are not completely integratable on one semiconductor chip for reasons of tolerance and because of the high resistance and capacitance values required. When sampling systems are used, the not negligible high-frequency spectral components require either a continuous-time prefilter (anti-aliasing filter) or a sufficiently high sampling frequency must be selected. With the presently increasing use of SC filters (switched capacity or filters), offset problems occur due to the analog operation. Moreover, under certain circumstances, the required high sampling rate may not be realizable, particularly when restrictions exist with respect to current consumption, for example, when a modem is fed by the central exchange battery.
If one thinks of using digital filters having a high sampling rate, a nonrecursive filter appears initially to be applicable since in the described case of filtering a binary signal, and in contrast to the recursive filter, "true" multiplication can be avoided and instead it is merely necessary to perform a weighting with 0 or 1. On the other hand, with the conventional type of nonrecursive filter, the transversal filter, as it is described, for example, by Samuel Stearns in Digital Signal Analysis, published by Hayden Book Company, Inc., Rochelle Park, N.J., 1975, Chapter 8, pages 102-120, the number of memory cells, and thus also the number of calculation operations to be performed per unit time, increases proportionally with the product of the sampling frequency and the system specific duration of the impulse response, and quickly reaches unrealistically high values.